In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into service areas or cell areas, which may also be referred to as a beam or a beam group, with each service area or cell area being served by a radio network node such as a radio access node e.g., a WI-FI access point or a radio base station (RBS), which in some networks may also be denoted, for example, a “NodeB” or “eNodeB”. A service area or cell area is a geographical area where radio coverage is provided by the radio network node. The radio network node communicates over an air interface operating on radio frequencies with the wireless device within range of the radio network node.
A Universal Mobile Telecommunications System (UMTS) is a third generation (3G) telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). Specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network.
3GPP 5G New Radio (NR) is the wireless standard that will become the foundation for the next generation of mobile networks. FIG. 1 depicts an overview of the downlink (DL) based active mode mobility (AMM) solution proposed for 3GPP 5G NR.
As shown in FIG. 1, a UE is served by the leftmost network node, i.e. the serving node 1, but is traveling in the direction towards the rightmost network node 2, depicted by the dashed arrow in the figure. The UE uses the best “home MRS” (Mobility Reference Signal) for coarse timing estimation and radio link quality monitoring and failure detection, denoted by the dot filled oval in the figure. Alternative names instead of MRS may be Active mode synch signal (AMSS), active mode reference signal or Channel State Information Reference Signal (CSI-RS).
In addition, the UE monitors a sparse periodic MRS from the serving network node 1 and compares it with similar periodic and sparse MRSs from potential target network nodes, e.g. the network node 2. When a target network node becomes relevant for a more detailed handover procedure additional dynamically configured home MRSs from the serving network node 1 and dynamically configured away MRSs from the target network node, e.g. the network node 2, may be activated.
The final handover decision is taken by the network and it is based on UE reports containing measurements of home MRSs and away MRSs.
An example of a proposed system information acquisition for 5G NR is depicted in FIG. 2. In the example each network node, which may also be referred as RBS, eNB, gNB, transmission and reception point (TRP), transmits a synchronization signal or a system signature signal (SS). Together with the SS each network node also transmits a physical broadcast channel (PBCH) containing some of the minimum system information that the UE need to access the network. This part of the minimum system information is denoted as master information block (MIB) in the figure. The transmission of SS and the physical broadcast channel (PBCH) containing the MIB is denoted with dot filled ovals in the figure.
By reading the MIB the UE receives information on how to receive the system information block (SIB) table. The SIB table may be transmitted using a broadcast format such as single frequency network (SFN) transmission and it is depicted with a dashed oval in the figure.
In addition to the minimum system information that is periodically broadcasted in the SS+MIB and in the SIB-table the UE may receive other system information e.g. by a dedicated transmission after initial access is established, depicted with an oval with label “Additional SI transmission” in the figure.
In 5G NR, which is designed to support high gain and dynamic beamforming, e.g. by means of utilizing hundreds of antenna elements at the base station, so called massive multiple-input-multiple-output (MIMO). It is therefore possible to maintain a connection with a UE despite that it is beyond an idle mode coverage or inactive mode coverage of the serving network node. The idle or inactive mode coverage, also referred to as SS coverage or SS broadcast area or system area coverage, is defined by the system information or system signature signal coverage, of a network node. That is with 5G NR, the possibility of beam formed data transmissions enables a UE to travel far away from its serving network node with a maintained radio quality. This means that the UE could move out of the SS broadcast area, or system area, of the serving network node, but still be connected to the serving network node. As long as the UE is still within a serving area or SS coverage of another network node in the network, this will not be a problem. As a UE that is dropped of connection due to some reason could easily reconnect to the network by retrieving the system information from that node. However, if the UE moves in to an inactive mode coverage hole, i.e. an area where none of the network nodes in the network broadcasts an SS, a UE that drops connection will not be able to reconnect to the network. This is because the UE cannot retrieve the information needed to do initial access.
FIG. 3 depicts an example of the problems identified above in prior art. As shown in FIG. 3, the UE is in an area where there is no SS coverage from any base stations, e.g. SS1, SS2, SS3 from the network nodes 31, 32, 33 cannot reach the UE. This mismatch in active mode coverage and inactive mode coverage will create a number of problems. The active mode coverage means that the UE has a Radio Resource Control (RRC) connection and is involved in transmission and reception of packet burst (note that it needs not transmit/receive constantly). Inactive/idle mode or state means that the UE has no RRC state in the network node, meaning that it has no radio bearers configured.
Firstly, this may cause a false sense of security to a user. A user with a UE that has an active mode connection to the network is under the impression that it can use the UE to make an emergency call, or some other important call, in the same position. For example, a user is travelling with its boat further out in the sea, i.e. is moving out of the Inactive Mode Coverage of the network. In case the UE would lose connection, the user would turn the boat around and go closer to the harbour again. However, as the UE is still connected to the network, the user is under an impression that it will be possible to call for help if it is needed. When the UE battery runs out, the UE drops and loses the connection to the network. The user has a backup battery to power on the UE, and tries to reconnect to the network. This is however not possible, as the UE cannot retrieve the information needed to do initial access.
Secondly, this may cause comparison problem. For example, two users with the same operator are standing next to each other in an area with no Inactive Mode Coverage of the network. User A has an active mode connection between the UE and the network which started before he is moving in to the area, and the active mode connection is now supported through beam forming. User B picks up the phone to make a call, but cannot connect since his UE is unable to retrieve any access information to the network. User B may get disappointed with the operator since the same service is not provided for both users, even though they are at the same location, have the same subscription and are paying an equal amount of money to the operator.